Process for the production of glass for a glass workpiece for use in drawing a glass light conducting fiber of a low attenuation

ABSTRACT

In a process for producing a glass blank or workpiece from which a glass fiber light conductor of a low attenuation is drawn, said workpiece being formed by providing a body on which amorphous glass powder is precipitated from a reaction gas after a chemical reaction and then transformed into a clear glass characterized by the reaction gas being passed over or through a powder composed of amorphous glass of the same type which is being precipitated from the reaction gas onto the body to filter impurities therefrom. Preferably, the amorphous powders of the filter are contained in a precipitator and are produced by also reacting the gas and collecting the powder in the precipitator.

BACKGROUND OF THE INVENTION

The present invention is directed to a process for the production of aglass workpiece from which a glass fiber light waveguide of a lowattenuation is drawn which process includes depositing amorphous glasspowders from a reactive gas, onto a surface of a body and subsequentlytransforming the amorphous glass powders into a glass film.

Glass fiber light waveguide conductors composed of extremely pure glassare used for optical communication transmission. A known process for theproduction of the glass fiber of this type is based upon providing awork piece or blank from which the fiber is then drawn. The workpiece orblank can be produced by a process in which an amorphous glass powder isprecipitated on a surface of a body and then subsequently melted to forma layer or film of glass. Such a process is known as a chemical vapordeposition process, and commonly known under the term "CVD Process". Inthis process, the glass body normally consists of a quartz glass tubewhose interior is contacted with the reaction gas. Normally a glass filmhaving a low index of refraction is first applied on the interior of thequartz glass tube whereupon a glass film having a higher index ofrefraction is subsequently applied. The reaction gases used for thispurpose are, for example, silicon tetrachloride and one or more othersuitable doping gases such as boron trichloride, germaniumtetrachloride, etc., and hydrogen. These reaction gases are caused toreact on the surface of the tube to form the amorphous oxide which ispartially or entirely precipitated on the wall surfaces of the tubewhere it is melted to form the glass film. The blank produced in thisway is subsequently collapsed so as to form a rod and the finalworkpiece or blank is then drawn into the form of a glass fiber. Lightconducting fibers having an attenuation value from approximately 2 to 3dB/km for a wavelength of 850 nm can be produced from such a blank.

In long distance traffic applications, in which the device itself andthe supply of intermediate amplifiers require a heavy financial outlay,for example, in a deep sea cable, it is desirable to produce fibershaving a lower attenuation value. It is also desirable that these fibersbe produced in a cost favorable process that has the lowest expenses.

A source of light loss along the fiber will consist fundamentally ofimpurities in the glass. These impurities may be hydroxyl groups forexample, .tbd.Si-OH, and metal oxides for example Fe₂ O₃. For thisreason, the reaction gases utilize a low humidity oxygen where the H₂O≦5 vppm and chlorides of a commercially available semiconductor qualityin which the SiHCl₃ in the SiCl₄ type is ≦0.2%. When an apparatus whichis subsequently impervious to gas is used and a dust filter is arrangedprior to the glass body acting as the reaction tube, the above mentionedattenuation values can be achieved. Under conditions of extreme purity,attenuation values of ≦1 dB/km for a wavelength of 850 nm and ≦0.5 dB/kmfor a wavelength of 1200 nm have in fact been achieved. (See M.Horiguchi et al; Electronics Letters, June 10, 1976, Vol. 12, No. 12;pages 310-312). In this case, in particular, it was ensured that no freeor chemically bonded hydrogen entered the reaction tube as a potentialsource for the OH groups. For this purpose, the following measures wereadopted. For example, cleansing of the chlorides of hydrogen compoundsfor example, silane and hydrogen chloride, by distillation; eliminationof water and hydrocarbons from the oxygen; drying of the chloridevessels; flushing with insert gases during the filling of the chloridevessels; and etc. However, all these measures require a considerableoutlay in time and are expensive and subject to disturbances.Furthermore, in practice, it is virtually unavoidable to keep thereaction mixture free of pollution. For example, leaks or corrosionphenomena in the pipeline system or due to flushing with water orparticles of dust during the rotation of the glass tube at the time ofthe glass deposition will cause a pollution of the mixture.

SUMMARY OF THE INVENTION

The present invention is directed to providing a process for producing aworkpiece or blank, which is subsequently drawn into a light conductingfiber having a low attenuation and the process enables producing theblank in a simple and problem free manner.

These objects are achieved by an improvement in a process for theproduction of a workpiece from which a glass fiber light waveguide of alow attenuation is drawn, said process comprising providing a body,providing a reaction gas, producing an amorphous glass powder from thereaction gas by a chemical reaction, precipitating the amorphous glasspowder on the body and subsequently transforming the precipitated powderinto a clear glass layer. The improvement comprises the step ofproviding the reaction gas including providing a filter composed of apowder of amorphous glass of an identical nature to the amorphous glassbeing precipitated on the body from the chemical reaction of thereaction gas and contacting the reaction gas with the powder of thefilter prior to the step of producing the amorphous glass powder by thechemical reaction.

The invention exploits the good absorption properties of a powder whichis composed of the amorphous glass which is of an identical nature tothat which is to be deposited from the reaction gas onto the body. Thus,the powder composed of the specific amorphous glass serves as anexcellent filter.

A particular advantageous and expedient embodiment of the process inaccordance with the present invention occurs when the step of providinga filter comprises providing a powder precipitator containing theamorphous glass powder of the filter and supplying a reaction gas to theprecipitator to produce the amorphous glass powder therein. In thisprocess, the powder composed of the amorphous glass, which acts as thefilter, is produced from the supplied reaction gas and is collected inthe powder precipitator. The quantity of the powder which acts as afilter can be very easily controlled by selective initiation andtermination of the chemical reaction in the precipitator.

In the event that the body consists of a hollow body such as a hollowglass tubular body whose interior surface is to be contacted withreaction gas, it is expedient if an outlet of the powder precipitator isconnected to the opening of the body and preferably the outlet of theprecipitator is fused to the end of the body. This is particularlyexpedient if the body is a glass tubular member and the precipitator isalso formed out of a glass body.

Preferably, the reaction gas contains at least one oxidizable siliconcompound and an oxidizing gas. If doped silicon dioxide layers are to beproduced, it is expedient for the reaction gas to also contain anoxidizable doping compound.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view illustrating an apparatus or a device for theimplementation of the process according to the present invention; and

FIG. 2 is a longitudinal cross-section of a powder precipitator having alabyrinth path in accordance with the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The principles of the present invention are particularly useful whenincorporated in a process utilizing a device generally illustrated inFIG. 1 for depositing glass films on an inner surface of a quartz glasstube 20. The device has a gas mixture system 1 and a glass turning lathe2.

For a detailed explanation of the invention, it is based on thefollowing example in which the quartz glass tube 20 has its interiorsupplied with a reaction gas and has glass films or layers formed on aninner wall or surface of the tube. It should be further assumed that thesilicon dioxide and doped oxides, for example P₂ O₅ or B₂ O₃, areprecipitated in a powder form from the reaction gas and these powdersare then melted until they produce a clear glass layer.

In the described example, good absorption properties of the amorphous,doped, or extremely pure silicon dioxide powders, which are producedfrom the gas phase, are exploited and are used to cleanse the reactiongas flow which, in this case, includes the dopant gas flow. For thispurpose, a powder precipitator 23 composed of glass is arranged toproceed the quartz glass tube 20 relative to the gas flow from the gasmixture system 1 and the precipitator has an outlet which is expedientlyfused to the end of the quartz glass tube 20. A heating zone proceedsthe precipitator 23 and a chemical reaction will occur in this zone toproduce amorphous SiO₂ -P₂ O₅ and SiO₂ -B₂ O₃ powders or other powderscomposed of the same gas mixture which are also used for the productionof the synthetic glass for both the casing or cladding and the core ofthe glass fiber waveguide.

The powder produced in this way from the gas phase will partially adhereto the precipitator 23. At the end of the precipitation procedure, theprecipitated powder serves as a surface active filter for the reactiongas when the portion of the gas which has not been decomposed in theprecipitator, flows into the quartz glass tube, which is to be coated.

The temperature in the heating zone is only reduced as to the extent asto prevent any fundamental oxidation of the chlorides. However traces ofH-containing compounds such as hydrocarbons and silanes react withoxygen in part or in whole to form polar, adhesive compounds. This willoccur at a temperature of approximately 600° C.

Now, a second traveling heating zone is used to deposit a film or layerof powder on the quartz glass tube 20 and the film of powder is meltedto form a clear glass film. This heating zone is created by a burner 21and is moved along the tube so that the reaction tube 20 is coateduniformly over a considerable length. Normally, an adequate overalllayer thickness is achieved by the use of a plurality of glass layers ofthe above described layer which can be achieved by repeatedprecipitation. During the return of the heating zone to a commencing orstarting position, the powder precipitator 23 is reloaded with freshsurface active powder by the heating of the first heating zone createdby the nozzle 22. The return time, which hitherto for was merely deadtime, now is used for cleansing the starting material. By varying thereturn time, it is easy to accomodate or dispense a quantity of powderin the precipitator.

Thus, the powder precipitator serves alternately to precipitate thesurface active oxide powder and the impurities. It is expedientlyconstructed in the form of a labyrinth in which the powder or theimpurities travel as a result of convection and diffusion towards thewalls where they adhere as a result of surface energy. In order toimprove the adhesion of the powder, the precipitator 23 can be cooled bymeans of an air flow from a nozzle 26. In contrast to the conventionaltechinques, pollution of the reaction tube by the cleansing means in theprecipitator is comparatively uncritical as these cleansing powders,which precipitate from the gas phase, are fundamentally identical to thepowder within the reaction tube 20.

The powder, which is not fused to form glass, is also precipitated atthe end 25 of the heated length of tube 20. In this way, after a fewprecipitation cycles, the tube contains a double sided protection fromimpurities which is constantly renewed. When an adequate thickness ofthe cladding glass layer has been reached, the precipitation of thelight conducting core glass is started. The all around protection of thecore glass with respect to impurities, which may diffuse into the tube,will remain in existence at the end of the precipitation process evenduring the collapsing process.

Light conducting fibers, which were drawn in a conventional fiberdrawing furnace from rods or workpieces produced in accordance with theinvention, exhibit distinctly lower attentuation values within thespectral range of from 500-1400 nm in comparison to comparable fibersproduced without the use of the powder precipitators.

The process in accordance with the invention can also be used forcleansing the reaction flow in the gas phase precipitation of oxidepowders or glass or amorphous oxide layers on the outside of forexample, cylindrical bodies or a flat disc. For example, the improvementcan be used in the deposition on a rod, the Verneuille CVD process andthe production of passivation layers on a semiconductor component.

The improvement can also be employed in processes, which use a differentform of energy supply to the reaction gas. For example, the improvementcan be used in processes, which precipitate from a gas flame such as theflame hydrolysis or processes that use plasma discharge such as a plasmaCVD process.

Furthermore, the improved process can also be used for the production ofextremely pure oxide powders from the gas phase as is required forexample in crucible melting processes for the production of lightconducting fibers. In any case, a preliminary precipitator is arrangedto be ahead of the actual reaction zone.

The exemplary embodiment of the invention has now been described indetail with regard to the schematic illustration of the apparatus inFIG. 1. This apparatus or device fundamentally consists of the two partsnamely a gas mixture system generally indicated by 1 and a glass turninglathe generally indicated at 2. A suitable gas mixture is produced inthe gas mixture system 1. This takes place in a conventional mannerusing dispensing valves and heating vessels for liquid, oxidizablestarting substance, as a rule chlorides, through which a flow of oxygenis passed by way of an oxidizing gas. For production of the casing orcladding glass, this apparatus can be used for example to produce a gasflow of a composition of 1100 Nml/min O₂, 90 Nml/min SiCl₄, 9 Nml/minBCl₃. For the production of the core glass 30 Nml/min GeCl₄ is added tothis gas flow.

The glass production takes place in the second part of the apparatus orthe glass turning lathe 2. The glass turning lathe has clamped therein acommercial quartz glass tube 20 of a 20 mm outer diameter and a 17 mminner diameter, whose interior is to be coated. The lathe 2 will rotatethe tube 20 about its longitudinal axis at an angular speed of 10revolutions/sec. The central part of this tube 20 is heated by means ofthe burner 21, which moves from a starting point or position at a speedof 2.5 mm/sec. to the right for a distance of 50 cm and creates a narrowheating zone on the tube 20, which has a temperature of approximately1600° C. After reaching the end of travel or right hand point, theburner 21 will be returned to the starting position at a speed of 17mm/sec. and at the time of return to the starting point the tube willremain relatively cool.

As soon as the burner 21 has reached the starting position, the nextheating cycle will commence. On the left-hand side of the heated lengthof the tube 20, the powder precipitator 23, which is composed of quartzglass, is fused onto the end of the tube. On the right-hand side thereis arranged an adjoining quartz tube 25, which has a larger diameter forexample 40 mm and which serves as an exhaust tube which leads to asuction system as indicated by arrow 27.

The reaction gas mixture is applied to the quartz glass tube from theleft-hand side through a rotary duct 10 which is impervious to the gas.Each time the burner 21 is returning to a starting position, a secondsmaller burner 22 is switched on directly proceeding the precipitator 23and locally heats the reaction gas mixture to a temperature at which thepowder formation from the gas phase commences. In the present example,this temperature is approximately 1400° C.

The powder thus formed is then precipitated as a white coating in theprecipitator 23. In order to increase its effectiveness, theprecipitator is cooled externally by a shower of air from a nozzle 26.

When the heating of the tube by means of the burner 21 commences, theoxygen supply for the burner 22 is disconnected. Thus, the tube leadinginto the precipitator is heated only by a hydrogen air flame whichproduces a temperature that is still adequate to decompose a compoundcontaining hydrogen, such as for example SiHCl₃, in the reaction gasflow. In the traveling heating zone, a conversion to oxide powder andthen into glass takes place. At the end of the heated length of thetube, such as at the tube 25, the powder is accumulated which will notbe completely melted into clear glass.

Following ten precipitation cycles using a BCl₃ /SiCl₄ /O₂ mixture, theproduction of the casing or cladding glass is furnished and GeCl₄ isadded to the glass flow so that the requisite jump in the index ofrefraction between the cladding glass and the core glass is reached.Without interrupting the experiment, fifty further precipitation cyclesare carried out. The tube is then collapsed to form a rod by a steppedreduction in the speed of the forward movement of the burner 21 and thusan associated increase in the temperature at the heating zone. Here thechloride supply is virtually disconnected and only a small concentrationof GeCl₄ is maintained in the gas flow in order to compensate forvaporization losses of GeO₂ on the inner wall of the coated tube.Shortly prior to the collapse of the tube to form the rod or workpiece,the gas flow is completely cut off. The rod can be then drawn to formthe fiber.

A comparison of the attenuation values of these fibers with the valuesof fibers produced by the same method or procedure but without the useof the powder precipitate indicates that the impurity content and thusthe light absorption can be reduced by means of the powderprecipitation.

As best illustrated in FIG. 2, the powder precipitator 23 has a bodywith an inlet and an outlet with a plurality of partitions or wallsegments 29 which are illustrated as being interdigitally arranged.Thus, the gas entering through the inlet as indicated by arrow 31 passesthrough the labyrinth or tortuous path prior to discharge from theoutlet as indicated by arrow 32.

Although various minor modifications may be suggested by those versed inthe art, it should be understood that we wish to embody within the scopeof the patent granted hereon, all such modifications as reasonably andproperly come within the scope of our contribution to the art.

We claim:
 1. In a process for the production of a workpiece from which aglass fiber light waveguide of a low attenuation is drawn, said processcomprising providing a body, providing a reaction gas, producing anamorphous glass powder from the reaction gas by a chemical reaction,precipitating said amorphous glass powder on the body, and subsequentlytransforming the precipitated powder into a clear glass layer, theimprovement comprising the step of providing the reaction gas includingthe steps of providing a filter composed of a powder of amorphous glassof an identical nature to the amorphous glass being precipitated on thebody from the chemical reaction of the reaction gas, and contacting thereaction gas with the powder of the filter prior to the step ofproducing the amorphous glass powder by the chemical reaction.
 2. In aprocess according to claim 1, wherein the step of providing the filtercontaining the powder of amorphous glass comprises providing a powderprecipitator containing the amorphous glass powder of the filter, andsupplying a reaction gas to the precipitator to produce that amorphousglass powder therein.
 3. In a process according to claim 2, wherein thestep of providing a body provides a hollow tubular body having aninterior surface being contacted by the reaction gas, said hollow bodybeing connected to an opening of the powder precipitator.
 4. In aprocess according to claim 3, wherein the opening of the powderprecipitator is fused to the end of the tubular body.
 5. In a processaccording to claim 2, wherein the powder precipitator is constructed inthe form of a labyrinth powder precipitator.
 6. In a process accordingto claim 1, wherein the reaction gas contains at least one oxidizablesilicon compound and an oxidizing gas.
 7. In a process according toclaim 6, wherein the reaction gas contains an oxidizable reducingcompound.